Deriving a tracking error signal from a time difference between detector signals

Abstract

An apparatus for scanning an optically readable record carrier 1, in which a light beam modulated by the information marks in a track on the record carrier is incident on a detection unit with four detectors. A reliable tracking error signal based on time differences between detector signals, is obtained for high-density recording by comparing digitized signals of the detectors with a delayed signal Td generated by a delay unit and based on the digitized signals. The time differences between the detector signals result in length differences between pulses in the comparing signals A and B, which pulses have a nominal length of Td. The comparing signals A and B are low-pass filtered and combined to generate the tracking error signal TE.

What is claimed is: 
     
       1. Apparatus comprising: means for scanning a track on a record carrier with a radiation beam; and   tracking means for controlling the position of the radiation beam in a direction transverse to a track direction on the basis of a tracking error signal and including: a detection unit including a multitude of detectors for receiving different portions of a radiation beam from the record carrier and for producing respective output signals upon receiving the corresponding radiation beam portions;   electronic circuit means for generating the tracking error signal depending on time differences between a first detector signal and a second detector signal, the first and the second detector signals being derived from the output signals of the detectors, the time differences of the first and second detector signals depending on time difference between receiving respective portions of the radiation beam at the corresponding detectors;   digitizing means for digitizing the first and second detector signals to provide first and second digitized signals;   delaying means for generating a delayed signal based on the first and/or second digitized signals; and   comparing means for comparing the delayed signal with the first and/or second digitized signals for generating the tracking error signal.     
     
     
       2. The apparatus of claim 1, in which the delayed signal is dependent on the digitized signal, either the first or the second digitized signal, that lags the other, and independent of the other digitized signal. 
     
     
       3. The apparatus of claim 1, in which the digitizing means include slicer means and AC coupling means for coupling the respective first or second signal to the slicer means for slicing the signal relative to a slicing level. 
     
     
       4. The apparatus of claim 1, in which the comparing means include: means for generating a first comparing signal depending on the difference between the delayed signal and the first digitized signal and a second comparing signal depending on the difference between the delayed signal and the second digitized signal;   low-pass filtering means for averaging the comparing signals; and   difference measuring means coupled to the outputs of the low pass filtering means for generating the tracking error signal.   
     
     
       5. The apparatus of claim 1, in which the detection unit includes: further digitizing means for digitizing a third and a fourth signal derived from the outputs of the detectors; and   second delaying means for generating a second delayed signal based on the third and/or fourth digitized signal; and   in which the comparing means include means for comparing the second delayed signal with the third and/or fourth digitized signals.   
     
     
       6. A detection unit comprising: a multitude of detectors for receiving a radiation beam from an information track on a record carrier and producing output signals;   electronic circuit means for generating a tracking error signal depending on time differences between a first detector signal and a second detector signal, the first and second detector signals being derived from the output signals of the detectors, the time differences of the first and second detector signals depending on time difference between receiving respective portions of the radiation beam at the corresponding detectors;   digitizing means for digitizing the first and the second detector signal to provide respective first and second digitized signals;   delaying means for generating a delayed signal based on the first and/or the second digitized signal; and   comparing means for comparing the delayed signal with the first and/or second digitized signals for generating the tracking error signal.   
     
     
       7. A difference detection unit for generating a difference signal depending on time differences between a first signal and a second signal, comprising: digitizing means for digitizing the first and the second signal;   delaying means for generating a delayed signal based on the first and/or the second digitized signal; and   comparing means for comparing the delayed signal with the first and/or second digitized signals for generating the difference signal.   
     
     
       8. Apparatus comprising: means for scanning a track on a record carrier with a radiation beam; and   tracking means for controlling the position of the radiation beam in a direction transverse to a track direction on the basis of a tracking error signal, and including: a detection unit including a multitude of detectors for receiving different respective portions of a radiation beam from the record carrier and for producing respective output signals upon receiving the corresponding radiation beam portions;   electronic circuit means for generating the tracking error signal depending on time differences between a first detector signal and a second detector signal, the first and the second detector signals being derived from the output signals of the detectors, the time differences of the first and second detector signals depending on time difference between receiving respective portions of the radiation beam at the corresponding detectors;   digitizing means for digitizing the first and second detector signals to provide first and second digitized signals;   delaying means for generating a delayed signal based on the first and/or second digitized signals; and   deriving a tracking error signal from the difference between the delayed signal and the first and/or second digitized signals.